1. Field of the Invention
The present invention relates to a projection optical system that is used mainly for a rear projector or the like using a DMD (digital micromirror device; made of Texas Instruments Incorporated of the United States), an LCD (liquid crystal display) or the like as a display device and has a long back focal distance and whose reduction side is substantially telecentric, and a projecting apparatus having the projection optical system.
2. Description of the Prior Art
Recently, high-performance and low-priced rear projectors having a DMD or an LCD as a display device have been spreading. By installing such a rear projector, for example, in a home, it is possible to see images received through BS digital broadcasting on a large screen or see movies on a large screen as a home theater.
Display devices used for such a rear projector have been becoming smaller in size and higher in resolution than before, and projection optical systems are required to be smaller in size, higher in performance and lower in price. Moreover, in recent years, in order that too much space is not taken up in a small room, there has been a great demand for thickness reduction of the rear projector, and a wider-angle projection optical system has been becoming necessary.
Therefore, for example, in a rear projector of Patent Document 1, by disposing an optical path bending member in the middle of the optical path of the light to be projected onto the screen and bending the optical path, the overall thickness and size of the rear projector are reduced.
Patent Document 1 is Japanese Laid-Open Patent Application No. 2003-248169 (see fourth embodiment, and FIGS. 7 and 8).
However, in the rear projector of the Patent Document 1, the space for disposing the optical path bending member, that is, the air equivalent distance between a first lens unit on the enlargement side (screen side) of the optical path bending member and a second lens unit on the reduction side of the optical path bending member is not appropriately set. Consequently, there are cases where a cost increase cannot be avoided when the thickness of the rear projector is reduced, so that the reduction in the thickness of the rear projector is hindered.
That is, when the air equivalent distance is larger than necessary, the diameter of the first lens unit is larger than necessary. Consequently, the cost of the first lens unit increases. Conversely, when the air equivalent distance is smaller than necessary, in order to prevent interference (collision) between the first lens unit and the second lens unit by the bend of the optical path, it is necessary to change the shape of these lens units (for example, to cut the lens elements at the surfaces parallel to the optical axis into an oval shape). Consequently, the cost of the projection optical system increases by the amount necessitated by such processing.